This adds PUT /api/5/agent/bridge and PUT /api/5/agent/gateway endpoints
for "menshen agents" running on bridges and gateways to make calls to in
order to register themselves.

These endpoints are authenticated with a x-menshen-agent-auth header
which should be the hex encoded HMAC of the body of the message.

When menshen receives a request to one of these endpoints, it will
either add that "resource" (gateway or bridge) to its "registry" or if
it determines that the resource already exists in the registry it will
update that resource's "last seen" property.

This also adds an optional "last seen cutoff" option to menshen where
menshen can choose whether it should show resources that haven't
received an update within the last N time. If that option is set to zero
then menshen won't take any last seen information into consideration
when returning resources to clients.